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A new picture of the early universe

Last Updated on Sunday, 02 May 2010 11:57

Published on Sunday, 27 February 2005 00:00

Physicists from the Universities of Cambridge and Manchester and the Instituto de Astrofisica de Canarias in Tenerife have released the first results of new high-precision observations of the relic radiation from the Big Bang, often called the cosmic microwave background or CMB. These observations have been made with a novel radio telescope called the Very Small Array (VSA) situated on the Mount Teide in Tenerife. The images show the beginnings of the formation of structure in the early Universe. From the properties of the image, scientists can obtain vital information on just what happened in the early universe and distinguish between competingcosmological theories.

Radiation from the Big Bang fireball has been travelling across the universe, cooling as space expands. Today, we see the faint relic radiation in all directions on the sky at a temperature of just 3 degrees centigrade above absolute zero, giving a picture of the universe when it was less than one 50,000th of its present age. Because galaxies must have formed out of the primeval fireball, astrophysicists have predicted that they will have left imprints in the radiation. Across the sky, there should be tiny variations in the temperature of the relic radiation. However, these ripples are very weak---only one 10,000th of a degree C.

During its first year of operation the VSA has observed three patches of sky, each some 8 x 8 degrees across. It can see detail down to one third of a degree, well matched to the typical size of interesting temperature variations. The VSA has 14 aerials, each somewhat akin to a satellite TV dish but only 15 cm across. The signals from each aerial are combined, forming an interferometric array - a technique pioneered by Cambridge physicists. The array is able to filter out unwanted terrestrial and atmospheric radiation allowing the the extremely faint CMB sky signal common to all the aerials to be detected. This approach allows high precision observations to be made at modest cost - the capital cost of the VSA was 2.6 million GBP. The performance of the VSA also results from using advanced receivers built at Manchester University and from the outstanding atmospheric conditions at the 2.4 km high Teide Observatory on Tenerife. The VSA can therefore measure specific, individual structures in the relic radiation with great precision.

A small number of other experiments have made similar observations. The different experiments work in different ways and face different challenges and sources of error; a key advantage of this diversity is that if their results agree, one can be confident that they are correct. One special strength of the VSA is that it is an interferometer array; another is that it is able to robustly remove the contaminating radiation from radiogalaxies and quasars that lie between us and the CMB relic radiation. The VSA results provide amazing confirmation of the current picture of the Universe.

The VSA observations of the CMB released today reveal the following properties of our Universe:

1) The curvature of space is close to zero -- we live in a spatially 'flat' universe.

2) The material in the universe is dominated by dark matter.

3) There is direct evidence for 'vacuum dark energy' which is currently not well understood, but is causing the universe to accelerate.

4) There are multiple peaks in the CMB power spectrum. This is direct evidence that all the structure in the universe today is due to microscopic quantum-mechanical fluctuations, inflated to astronomical size in the very early universe.

The VSA is a collaborative project between the Astrophysics Group at Cambridge University's Cavendish Laboratory, Manchester University's Jodrell Bank Observatory, and the Instituto de Astrofi'sica de Canarias (IAC) in Tenerife. The project is funded by the UK Particle Physics and Astronomy Research Council and the IAC.

Background notes

The cosmic microwave background

The cosmic microwave background radiation was discovered in 1965 by American physicists Arno Penzias and Robert Wilson (who received the Nobel Prize for the work). It is a faint radio radiation which fills the entire universe, and thus appears to come from all directions in the sky. It is believed to be the relic of the hot Big Bang phase of the universe, when the entire universe was roughly the temperature of the surface of the Sun. The expansion of the universe has since cooled the radiation down to a temperature of just under 3 degrees above absolute zero (ie -270 deg C). There are tiny variations in this temperature, of a few parts in 100,000, which were first discovered by the NASA satellite COBE in 1992. These are due the tiny fluctuations in density of the universe which have since collapsed under gravity to form all the structures (galaxies and stars) in the universe. These density fluctuations are believed to be quantum fluctuations, blown up to astronomical size by a process in the very early universe called 'inflation'.

The power spectrum of the cosmic microwave background

Astronomers describe the fluctuations in the cosmic microwave background by its power spectrum. This is a graph of the strength of the fluctuations versus their angular size. Theories of the universe can predict the shape of this graph in detail, and the theories are tested by comparing the observed power spectrum to the predictions. An important prediction of the favoured class of theories, based on the theory of inflation, is that the power spectrum should show multiple peaks. These are due to coherent oscillations(sound waves) in the hot plasma of the early universe, driven by quantum fluctuations that had been vastly enlarged by the process of inflation.

Flatness of the Universe

Einstein proposed in his General Theory of Relativity in 1915 that matter and energy cause space to become curved. In curved space geometry works differently to normal flat (Euclidean) geometry: the angles of a triangle don't add up to 180 degrees. Einstein showed that the curvature of the entire universe depends on the amount of matter and energy in it. If there is relatively little matter/energy, the universe is negatively curved (like the surface of the bell of a trumpet or the stem of a wine glass). If thereis a lot of matter/energy, space is positively curved (like the surface of a ball) - this also means the universe is finite in size. If the amount of matter/energy is just right, space is flat, and traditional school geometry does apply. Observations of the the MB measure the curvature of space by effectively constructing a triangle between the observer and the edge of the observable universe, and measuring its angles. These measurements are showing that space is indeed flat.

Dark Matter and Dark Energy

Astronomers have long known that there must be another type of matter in the universe besides the ordinary matter that the stars and planets are made of. This matter is detected by its gravitational effects, but what form it takes is a mystery; some type of new heavy subatomic particle is usually assumed, and given the name dark matter. Einstein's General Theory of Relativity also allows for the existence of dark energy (also called the Cosmological Constant). This is a property of empty space that causes the universe to expand more and more rapidly. Long thought to be a mathematical curiosity,it now turns out that the dark energy is real; the accelerating expansion was discovered in the last few years by observations of distant supernovae. Now the observations of the CMB have confirmed this. Both dark matter and dark energy contribute to the flatness of the universe, but the amount of dark matter can also be measured by combining the CMB measurements with measurements of the Hubble Constant (the expansion rate of the universe). There is not enough dark matter to make the universe flat, so there must be a contribution from dark energy too. The nature of the dark energy is not at all understood.

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